Pentagon chief Mattis says Pyongyang's actions could lead to 'the end of its regime and the destruction of its people'.

US Defence Secretary Jim Mattis has responded to North Korea's threat of targeting the US Pacific territory of Guam with missiles, saying Pyongyang's move would lead to "the end of its regime and the destruction of its people".

Mattis said in a statement on Wednesday that North Korea "would lose any arms race or conflict in initiates".

"The DPRK must choose to stop isolating itself and stand down its pursuit of nuclear weapons," he said, using the North's official name, the Democratic People's Republic of Korea.

Earlier on Wednesday, a spokesman for the Korean People's Army said in a statement carried by North Korea's state-run KCNA news agency that the Guam attack plan would be "put into practice in a multi-current and consecutive way any moment" after leader Kim Jong-un made a decision.

In another statement, quoting a different military spokesman, North Korea also said it could carry out a pre-emptive operation if the US showed signs of provocation.

On Tuesday, US President Donald Trump threatened North Korea "with fire and fury like the world has never seen" after reports suggested the communist country has mastered one of the final hurdles to the ability to attack the US with a nuclear missile.

The nuclear advances were detailed in an official Japanese assessment and a Washington Post story that cited US intelligence officials and a confidential Defense Intelligence Agency report. The US now puts the North Korean arsenal at up to 60 nuclear weapons, more than double most assessments by independent experts, according to the Post's reporting.

"North Korea had best not make any more threats to the United States," said a stern-looking Trump, seated with his arms crossed and with his wife beside him, at his golf course in Bedminster, New Jersey.

The remarks appeared scripted, with Trump glancing at a paper in front of him. They evoked President Harry Truman's announcement of the US atomic bomb dropped on Hiroshima, Japan, in 1945, in which he warned of "a rain of ruin from the air, the like of which has never been seen on this earth".

Guam hosts strategic US military installations - including both a naval and an air force base - and is more than 3,400km from North Korea. Its population is around 160,000 - but only about 6,000 US troops are currently stationed there.

Robert Kelly, professor of political science at Pusan National University, SAIDthat both North Korea and Trump were almost certainly exaggerating in their respective rhetoric, as a conflict would be disastrous - and that Trump was likely stepping up the rhetoric to pressure China into taking action to curb North Korea's behaviour.

"If the North Koreans were to strike Guam and kill a lot of Americans it would lead to a war which would destroy North Korea, so my guess is this is bluffing on both sides," he said.

"But it certainly doesn't help because it raises the possibility of miscalculation and misperception," added Kelly.